Since the association of rabbit muscle phosphofructokinase is apparently pH dependent, between pH 6.0 and 8.0, this variable will be carefully followed by sedimentation velocity, giving information on the difference between the affinities for H ion monomers and aggregates obtained by applying the linked function theory expressed by Wyman (Adv. Protein Chem. 19, 224, 1964). On the other hand, by keeping the solution at a constant pH and the activity of one of the solute species is varied then the equation becomes an expression for the differential of the ligand associated with monomer and aggregates. The ligands of interest are ATP, citrate and the fructose phosphates.